Flexible pipes for transporting hydrocarbons generally comprise from the outside to the inside of the pipe:                an external polymeric sheath for protecting the whole of the pipe and notably for preventing seawater from penetrating into its thickness,        tensile armor plies,        a pressure vault,        a internal sealing polymeric sheath, and        optionally a metal carcass.        
If the pipe comprises a metal carcass, it is said to be with a rough bore. If the pipe is without any metal carcass, it is said to be with a smooth bore. Generally, for transporting hydrocarbons, a pipe comprising a carcass is preferred while a pipe without any carcass will be suitable for transporting water and/or pressurized steam.
The metal carcass and the pressure vault consist of longitudinal elements wound with a short pitch, and they give the pipe its resistance to radial forces while the tensile armor plies consist of metal wires wound with long pitches in order to spread out axial forces.
The nature, the number, the dimensioning and the organization of the layers forming the flexible pipes are essentially related to their conditions of use and of installation. The pipes may comprise additional layers to those mentioned above.
In the present application, the notion of winding with a short pitch designates any helicoidal winding with a helix angle close to 90°, typically comprised between 75° and 90°. The notion of winding with a long pitch, as for it, covers helix angles of less than 55°, typically comprised between 25° and 55° for the armor plies.
These flexible pipes are notably suitable for transporting fluids, notably hydrocarbons in sea beds and this at great depths. More specifically, they are said to be of the unbonded type and they are thus described in the normative documents published by the American Petroleum Institute (API), API 17J and API RP 17B.
The flexible pipes may be used at great depth, typically down to 2,500 meters deep. They allow transport of fluids, notably of hydrocarbons, having a temperature attaining 130° C. typically and which may even exceed 150° C. and an internal pressure which may attain about 1,000 bars, or even 1,500 bars.
The constitutive material of the internal sealing polymeric sheath has to be chemically stable and capable of mechanically withstanding the transported fluid and its characteristics (composition, temperature and pressure). This material should combine ductility, durability (generally the pipe should have a lifetime of at least 20 years), mechanical strength, heat and pressure resistance characteristics. The materials should notably be chemically inert with regard to the chemical compounds making up the transported fluid.
Pipes comprising a sealing polymeric sheath based on polymer, notably on polyamide or polyethylene, may notably be used. However, the thermomechanical properties of polymers, notably of polyamides or polyethylenes, under the conditions of use mentioned earlier (high temperature and pressure, high acidity and presence of water) may be significantly reduced. In particular, polyamides may be degraded by hydrolysis. Further, it is difficult to use polyethylenes at temperatures above 65° C. Thus, many studies have been reported for attempting to improve these properties, in particular for improving their resistances to creep and to tension or compression. For this, the mobility of the polymer chains relatively to each other should be reduced by longer and/or more cross-linked polymer chains.
Thus, international application WO2007/078038 describes a thermoplastic resin, notably in polyamide, comprising nanoparticles of metal oxide, the surface of which is covered with silanes, allowing binding between the polyamide and the nanoparticles.
International application WO93/05086 describes a method for increasing the molecular mass of polyamides by an amidation process applying a catalyst of the phosphonic acid type in the presence of titanium dioxide comprising alumina.
International application WO2008/113362 describes a flexible pipe comprising a polymeric layer, notably based on polyamide, comprising a cationic clay, notably of the smectite type.
International application WO 03/078134 describes a flexible pipe comprising a layer based on polyethylene cross-linked with a peroxide.
International application WO 91/19924 describes a flexible pipe comprising a layer based on polyethylene cross-linked with a silane.
The mechanical properties of the modified polyamide or polyethylenes of these applications are affected. Nevertheless, these properties further have to be improved in order to allow transport of fluids under the extreme conditions mentioned above.